Continuous chemical milling process



June 16, 1959 s. A. HAYS 2,890,944

CONTINUOUS CHEMICAL MILLING PROCESS Filed May 25, 1956 I HII'IHIII V v I6 INVENTOR.

STEPHEN A. HAYS ATTORNEY United States Patent "CONTINUOUS CHEMICAL MILLING PROCESS Stephen A. Hays, La Miratla, Calih, assign'or to North American Aviation, Inc.

Application May 25, 1956, Serial No. 587,418

17 Claims. (CI. 41-42) This invention relates to improvements in the process for chemically milling group VIII metals and alloys thereof. More specifically, this invention relates to a chemically milling process for metals which can be controlled to produce finished products of uniform quality.

Processes have recently been developed for chemically milling steel. These processes involve contacting a steel workpiece with an acid chemical milling solution for a period of time suificient to remove a desired amount of metal from the exposed surface of the steel workpiece. The method employed in the past has consisted of a batch process, that is, an acid solution of a particular strength was made up and steel workpieces subjected to its action for a given period of time. During this period, the strength of the solution was gradually depleted. When the strength of the solution became too low for chemical milling, the solution was discarded and a new solution was made up. The prior art processes, however, contain a number ,of disadvantages. One disadvantage is that the'elfectiveness of the. milling solution is often exhausted in a short period of time requiring interuption .of the process while new solutions areprepared. On a commercial; scale, the cost of such a procedure is often prohibitive. A characteristic disadvantage of the batch process is that thestrength ofthe solutionis very high at the beginningof the, chemicalmilling and decreases to minimum vefitectiveness as the milling progresses. This makes it very diflicult tocontrol not only the rateof the chemical tmilling, but. also the quality -of the chemically milled surface. For instance, whenthe acid concentration of the solution'is high, a veryrough surface is obtained on the part exposed tothe solution. Likewise when the solution has become, run downor too low in acid concentration,.,a poor. or rough surface is also. ob-

tained. Still another problem in the. prior art batch process is the disposal of. the .spent chemical milling solutions.

The acid, I nitrate and metal content of the. waste solutions prohibit direct disposal in sewage systems. kaline neutralization is costly and still leaves a high nitrate solution' which, also, may not be disposed of through the sewagesystems. It is thus seen that a need exists. for a process of chemicallymilling metals such as steel and steel alloys which not only can be controlled object iahthisinvention to provide a chemical milling process by the use of whichthe quality of the chemically milled surface may be controlled. Another object of this 2,890,944 Patented June 16, 1959 invention is to provide a process which'climinates waste disposal problems. The aboveand other objects of invention will become more apparent from the discussion that follows.

The above andother-objectsof the invention are accomplished by a continuous process of chemically milling workpieces of group VIII metals and alloys thereof comprising exposing a workpiece to the etching action of a chemical milling bath initially containing dissolved "phosphorus and metal in a predetermined'weight ratio, wherein the phosphorus is present in the form of soluble phosphates, withdrawing partially spent portions of the bath containing the dissolved metal and phosphorus, and adding phosphorus inthe form of solublephosphates to the bath inan amount substantially equivalent to the phosphorus removed; in the partially spent portions; By carrying out the process in this manner, theconcentration of phosphorusand metalin solution'is thus maintained withina predetermined range of values and provides a methodfor obtaining workpieces having smooth chemically milled surfaces from which the metal has been removed to aneven depth. 3

The steel and steel alloys that can be chemically milled by the process of this invention includes metal alloys having very low percentage of steel by weight. For example, the process can be applied to milling Inco'nel X which contains only 7 weight percent iron andf weight percent nickel, together with other elements. The complete composition of Inconel"X is given in Bulletin T-.7 entitled Engineering Properties of Inconel and Inconel X, issued by the International Nickel Company, Inc, New York, in June 1953. It can also be applied to milling Inva'r which coritains 36 weight percent nickel and 64 weight percent steel. Generally, the process is applicable to chemically milling the metals of group VIII of the periodic table of elements and their alloys. i l

The dissolved phosphorus is present in the form of soluble phosphates such as phosphoricacid and metal phosphates'including soluble metal hydrogen phosphates. Examples of such phosphates are phosphoric acid, ferric phosphate'and its hydrates, the hydrogenph o'sphates of iron such as Fe (HP O and Fe(II PO chromic phosphates andits hydrates, chromic hydrogen phosphates, nickel phosphate, nickel hydrogenpho'sphates, etc. 1 The dissolved metal is pnesent'in the form of metal salts such as the metal phosphates named above or the soluble nitrate and chloride salts of the metals. Hence, the dissolved metal will be presentin solution "in the form of metal ions. The kind of metal in solution'will depend to a large extent on the composition of the metal workpieces subjected tothe action of the chemical milling bath and includes metals such as iron, nickel, chromium, cobalt, as well as small amounts of various metal impurities found in steel and its alloys. l

The single figure of the drawing represents a fiow sheet of the process. v i

Am embodiment of this invention comprises precipitating metal phosphates from the partially spent portions which have been withdrawn from the chemical milling bath, leaving an effluent or liquid phase containing metal hydrogen phosphates, and returning the liquid phase which contains the metal hydrogen phosphates back to the bath. The only amount of phosphorus in the form of soluble phosphates that need be added in this case is that equivalent to the amount precipitated as metal phosphates from the withdrawn portion. Since the. process is applicable to the chemical milling of steel and its alloys, the metals going into the solution will include not only iron but also other metals such as chromium, nickel, cobalt, etc. Hence the metal phosphates which chromium, etc.

are precipitated from the withdrawn partially spent portions of the chemical milling bath comprise a mixture of the phosphates of the various-metals of which the workpiece consists, i.e.,' phosphates of iron, nickel, cobalt, In like manner the eflluent which is returned to the milling bath will contain hydrogen phosphates of these metals.

Ingredients of the bath which are lost by reaction and evaporation, or removed in the withdrawn portions, are replaced by the additions of the respective ingredients to the bath. v

The purpose of the precipitation is to remove excess metal from the milling solution. Returning the effluent to the milling bath provides for the utilization of all the other components of the-milling solution. Thus the process provides a chemical milling method which not only results in controlled removal of'metal from the exposed surface of a workpiece, but also minimizes waste of bath components and results only in a metal phosphate by-product.

The other components of the chemical milling bath are comprised of hydrohalic acid and/or nitric acid together with 'water. Small amounts of other components including inorganic and organic acids such as sulfuric acid, nitrous acid, acetic and other organic acids may also be present either as impurities or they may be added to modify the chemical milling solution.

The dissolved phosphorus-to-metal weight ratio in th milling solution can vary from about 0.23 to about 1.50. When the ratio is maintained within this range, it is found that the process readily lends itself to control of the rate at which the metal is chemically removed from v the surface exposed to the milling solution. The metal is removed to a uniform depth over the entire exposed surface. Also, a smooth chemically milled surface is obtained regardless of the amount of metal removed per unit area. Since the ferric and other metal phosphates which are precipitated from the effluent are the only byproducts ofthe process, the previously encountered acid disposal problem is overcome. Furthermore, since phosphates are well-known articles of commerce, the process of. this invention not only eliminates the wastes inherent in prior art processes but also produces a usable byproduct.

An embodiment in the practice of this invention is the i use of a combination of nitric, and hydrohalic acid such as hydrochloric and/or hydrofluoric acid, in the milling solution together with phosphoric acid and dissolved metal. When employing such a solution for chemical milling of steel and steel alloy workpieces, the total acid content of the chemical milling solution as well as the amount of halide, nitrate, phosphate and metal ions should also be kept within certain ranges in order to obtain a chemically etched surface of good quality.

Therefore, an embodiment of this invention is a process for chemically milling steel and steel alloy workpieces comprising contacting said workpieces in a milling vessel with an acid chemical milling bath or solution containing 3.5-123 grams per liter of acidic hydrogen, 35.5-302 grams per liter of chloride ion, 80.5279 gramsper liter of nitrate ion, 43-172 grams per liter of phosphorus in the form of soluble phosphates, 30-200 grams per liter of dissolved metal, the balance being substantially water,

ratio; precipitating metal phosphates from the withdrawn portions leavinga liquid phase containing metal hydrogen phosphates; returning the liquid phase containing metal L hydrogen phosphates to the milling bath, and adding phosphorusin 'the. fornrof soluble phosphates to the bath "in an amount substantially equivalentlto the phosphorus removed in the form of the metal phosphate precipitate. One method of precipitating the metal phosphates from the Withdrawn portion is to first distill the withdrawn portion to produce a distillate of water, a distillate of hydrochloric plus nitric acids, and a residue containing phosphates of metal. Water is then added to residue to precipitate metal phosphates leaving an aqueous solution containing hydrogen phosphates of metal. The solution is separated from the precipitate and returned to the milling vessel. The solution or eflluent. may be condensed by distilling or evaporating to remove excess water, if any.

Small amounts of the various acids and water are usually lost invcarrying out the process by evaporation, distillation and reaction. To compensate for this, additions of the various components of the bath may be made from time to time in order to replace those lost.

The general method of carrying out the process of this invention may be more readily explained .by reference to the accompanying diagram. In this diagram a steel workpiece 1 has its edge 2, as Wellas the reverse side of the workpiece, masked with a suitable acid resistant material. The workpiece 1 is suspended by. means of lines 3 which are likewise coated with a suitable acid resistant maskant, in a chemical milling solution 4 contained in a milling vessel 5. The milling vessel is constructed of an acid resistant material and is equipped with heating and cooling means as well as a'temperature control means not shown in the diagram. A portion of the acid milling solution which contains phosphoric acids and dissolved metal is withdrawn through line 6 by cracking valve 7 and is admitted into a distillation apparatus 8. This withdrawn portion is progressively distilled to separate water and acids other than phosphoric such as hydrohalic and nitric acids, leaving a residue containing phosphates of iron and other metals. The residue may also contain some phosphoric acid which has not been used up in the formation of phosphates of iron. The water is taken oif through line 9, and collected in water container 11. The water is taken off at temperatures of up to about 103 C. and the rate to which it is taken off is controlled by the valve 10. When hydrohalic and nitric acids are employed in the bath, they are also distilled oif at temperatures of up to about C. and are taken off through line 12, controlled by valve 13 and condensed in condenser 14. From the condenser, these acids are readmitted to the milling vessel through valve 15, thus completing the cycle for the hydrohalic and nitric acids. The residue containing phosphates of iron and other metals is withdrawn from the distilling apparatus through valve 16 and admitted to a precipitation vessel 17. Water, supplied from an outside source 39 to tank 11, is admitted through valve 19 in line 18 to the precipitation vessel 17. Ferric phosphate and other metal phosphates precipitate from the diluted solution, leaving hydrogen phosphates of iron and other metals in the solution. To aid in the precipitation of metal phosphates, heat can be applied to bring the contents of the precipitation vessel to, and maintain them at, the boiling point. The slurry of metal phosphate precipitate and metal hydrogen phosphates is next passed through valve 20 and filter 21. The metal phosphate precipitate is removed from the solution and the aqueous metal hydrogen phosphates, together with any free phosphoric acid'that may be present, are conducted through valve 23 in line 22 into a concentration vessel 24; Heat is applied to the concentration vessel and its contents to distill or evaporate 01f excess amounts of water. The water distillate is taken ofi through valve 26 in line 25 and condensed and collected in the container 11. Alternatively, the water vapor may be vented to the atmosphere. The concentrated metal hydrogen phosphates are withdrawn from the concentration vessel through valve 27 and are conducted with the aid of pump 28 through line 29 back to the chemical milling vessel 5. Thus, by'meansof this process, metal removed from the workpiece 1 is precipitated metal phosphates, while tion are returned to the milling vessel. To replace the phosphate removed in the precipitate, an equivalent amount of phosphoric acid is added to the milling vessel from the container 30 through valve 31 and line Any water, hydrohalic acid and nitric acid which is lost in the various steps of the process or by evaporation and/or reaction; is replaced from the containers 32, 34 and 36 by manipulating the proper valves 33, 35 and 37 connected to the line 38 which leads to the milling vessel. Container 34 marked HX contains hydrohalic acid such as hydrochloric acid and/ or hydrofluoric acid.

The rate at which the chemical milling solution is withdrawn through line 7 depends on the amount of metal surface exposed to access of the chemical milling solution. The rate of distillation of the withdrawn portion in the distilling apparatus 8 is adjusted to handle the amount of solution withdrawn. When the temperature of the acid vapors taken off reaches about 130 C., substantially all of the hydrochloric and nitric acids have been removed from the withdrawn portion. The residue containing the phosphates of iron and other metals may be continuously withdrawn through valve 16 to prevent a buildup in the distillation apparatus. This is preferred to an intermittent withdrawal although the latter is also practiced with good success. The precipitation in the vessel 17 may, likewise, be carried on either batchwise or continuously. The continuous process is preferred where a large amount of metal is being dissolved per unit volume of chemical milling solution per unit time since it is necessary to continuously withdraw the excess iron from the solution and to replace the phosphoric acid which is used up. a

The point at which the excess water has been removed from the filtrate which contains the hydrogen phosphates of iron in concentrator 24, may be determined by the temperature of the distillate. When the temperature of the distillate has reached substantially 105 C., the metal hydrogen phosphates are suificiently concentrated for return to the chemical milling vessel. Alternatively, the milling bath level, which varies with evaporation, deletions through line 6, and additions through line 38, may be maintained by adding solution fromthe concentrator 24 regardless of its concentration. Then, only the excess-water need be evaporated from the solution containing the metal hydrogen phosphates. This may be accomplished by maintaining a maximum level in the concentrator. A float-controlled heater switch in the concentrator is one way of controlling the evaporation of the excess water.

The acid content of the milling solution is checked periodically by analysis for nitrate and chloride ion. The chloride ion is determined by a modified argentometric titration according to the method given in Volumetric Analysis, volume II, by Koltotf and Stenger, 1942 edition, page 242, Interscience Publishers, Inc., New York, with the modification consisting of the use of an electrometric end point. The nitrate ion determination is preceded by a basic precipitation of the metal from the sample. Following this, an analysis of the filtrate is conducted according to the method described in Scotts Standard Method of Chemical Analysis, 5th edition, by N. H. Furman, published by D. Van Nostrand of New York, vol. I, page 649.

The amount of phosphoric acid to be added can be determined from the weight of metal phosphates removed as precipitate. If desired, the amount of phosphate in the milling solution can also be determined by chemical analysis according to the method described in volume I of Scotts Standard Method of Chemical Analysis, 5th edition, page 697.

The following nonlimiting examples illustrate the processes of this invention in which the acidic hydrogen is defined as the hydrogens from the hydrohalic acid, nitric Cit 6 acid and all three hydrogens from the phosphoric acid added to the bath, less'the amount liberated due to reaction. The amount liberated is measured by the equivalence of metal dissolved.

. EXAMPLE I To the milling vessel of an apparatus'of the type shown in the figure, was added ,water, phosphoric acid, nitric acid and hydrochloric acidin amounts such as to make up a solution containing 113 grams per 'liter of phos phorus in the form of soluble phosphate, 61 grams per liter of chloride ion and 108 grams per liter of nitrate ion. Asarnple of 17-7 pH steel containing 17 weight percent chromium and 7 weight percent nickel was then suspended in the chemical milling solution and the solution allowed to act thereon until the concentration of metal in solution reached 106 grams per liter as determined by the loss of weight of the steel sample. During the build up of the metal in solution, hydrochloric acid and nitric acid were added in amounts equivalent to 178 grams per liter of N0 ion and 43 grams per liter of, Cl ion. Some nitric and hydrochloric acids were lost due to reactions and evaporation. The concentrations of N0 ion and Cl" at this point were 148 grams and 92 grams per liter respectively. A 174' pH steel workpiece with masked edges was then suspended in the vessel allowing the chemical milling solution to act on the free surface. As the chemical milling of the workpiece progressed, a portion of the milling solution was continuously withdrawn and processed according to the procedure described hereinabove, with return of hydrochloric and nitric acids and of hydrogen phosphates of iron, chromium and nickel. The nitric, hydrochloric and phosphoric acids used up in the process were replaced by adidtions of appropriate amounts of the respective reagents. 'The workpiece was chemically milled to a depth of .070 inch at the. rate of .020, inch per hour. The. workpiece was then removed and washed with water. The surface of .the chemically milled portion ofthe workpiece was smooth and chemically milled to an even depth. The weight ratio of phosphorus-to-iron in this example was 1.08, while the concentration of acidic hydrogen was about 9.6 gramsper liter.

Equally good results were obtained when the chemical milling solution waspreparecl by adding .204 grams phosphoric acid reagent containing percent acid by weight, 256 grams of hydrochloric acid reagent containing 37 percent acid by weight, 214 grams of nitric acid reagent containing 70 weight percent acid and 354 grams of ferric orthophosphate dihydrate per each liter of chemical milling solution to be prepared, the balance beingsubstantially water. Thus, the weight ratio in solution of phosphorus-to-iron of 1.08 was directly obtained without requiring the solution to act on a piece of steel until the desired amount of metal had been dissolved.

EXAMPLE II The procedure of Example I is repeated with a solution containing 30 grams per liter of iron, 43 grams per liter of phosphorus in the form of phosphoric acid, 35.5 grams per liter of chloride ion and 110 grams per liter of nitrate ion. The phosphorus-to-metal weight ratio in solution in this case is 1.50 and the acidic hydrogen concentration is 5.5 grams per liter. A workpiece of 17- -7 pH steel exposed to this chemical milling solution for a period of four hours is chemically milled to a depth of substantially .080 inch, producing an even and smooth surface.

EXAMPLE III A chemical milling solution containing 200 grams of iron per 'liter, 172 grams of phosphorus per liter in the form of phosphoric acid, 142 grams of chloride ion per liter and grams of nitrate ion per liter is employed in the process described in Example I in milling workpiece of 316 Cres steel for a period of four hours to produce chemically milled surfaces of smooth and even quality. The weight ratio of phosphorus-to-metal in solution in this case is 0.90 and the acidic hydrogen concentration is 12.3 grams per liter.. The Cres steel in this case constance, No. 11 in the table ilu'strates the undesirable effect that an excess of acidic hydrogen has on the chemically milled surfaces"- No; 12 illustrates the bad effect that an excess of nitrate ion has in milling solu- 'weight percent nickel, together with small amounts of other impurities. The 4130 steel contains about 97 .weight percent iron, 1 weight percent chromium, 0.5

weight percent manganese, 0.3 weight percent carbon and small amounts of other impurities. The Inconel X contains about 7 weight percent iron, 75 weight percent nickel and 15 weight percent chromium, plus small amounts of other elements.

The above examples illustrate the results obtained by maintaining the composition of the chemical milling solution constant at certain predetermined values of the concentration of the various components. Tl1e phosphorus, chloride ion, nitrate ion and acidic hydrogen concentrations were given in grams per-liter. In each case, the balance consisted essentially of Water. In the cases in which acceptable results were obtained, the weight ratio of phosphorus-to-metal in solution varied from 0.23 to 1.50; the acidic hydrogen varied from 3.5 to 12.3; the chloride ion concentration varied from grams per liter to 302 grams per liter, and the nitrate ion concentration varied from 80.5 grams per liter to 290 gramsper liter.

When the-values of either of the weight ratio of 'phosphorusto-metal, or, the acidic hydrogen or nitrate ion concentration varied beyond these limits, it was found that poor chemically milled surfaces resulted. For intains about 18 weight percent chromium and about 13 5 tions. 1' No. 13, "on the'other'han'd, illustrates the disweight percent nickel. advantages of using a solution having too little dissolved X l I I iron as compared to the amountof phosphorus in solu- E PLE IV tion. Thus, it is seen that, a weight ratio of 1.79 of A chemical milling bath containing 200 grams of iron, phosphorus-to-metal istoo high. 43 grams of phosphorus in the form of phosphoric acid, 10 The above examples thus illustrate the advantage that 302 grams of chloride ion and 80.5 grams of nitrate ion is obtained by using the process of this invention in per liter of solution is employed to mill a workpiece of chemically milling steel and steel alloy workpieces. By 17-7 pH steel. ,The exposed surface is chemically milled keeping the chloride'or other halide ion, the nitrate ion to a uniform depth. The weight ratio of phosphorus-toand the weight ratio of phosphorus-to-metal in solution metal in solution in this case is 0.23, while the acidic 15 within certain predetermined limits, good chemically hydrogen concentrations are 3.5 grams per liter. milled surfaces can be obtained evenwhen the amount Other nonlimiting examples of composition of chemof metal removed from thesurfac'e of the workpiece is ical milling solutions of this invention employed for chemconsiderable. This uniformity of chemically milled surically milling steel and steel alloy workpieces are given 0 faces is of extreme importance in the manufacture of .in the following table. 2 structural metal parts as, for example, in' the manufac- T able I.C0ncentration in grams/liter I i PRODUCTS ACCEPTABLE D Acidic Rate of Time, No. Workplece H 01" NO; P Metal Wt. milling, Hrs. Remarks Ratio inch/hr.

1 17-7 pH Steel. 62 220 113 191 0:50 .020 1.0 Smoofth uniform do 8.5 192 149 113 190 0. 50 .020 1.0 ifi Y d0 10.6 50 124 158 1.05 012 1.3 Do. d0 5. 9 163 45 112 0. 4O 037 0. 6 DO. do 0.0 112 105 113 110 1.03 .020 1.5 Do. 1o-. 10.6 96 180 113 110 1.03 .047 1.6 Do. do 9.0 92 114 113 106 1.07 .020 3.5 Do. do. as 71 110 113 91 1.24 .023 4.0 D0. 4130 Steel..- 11.3 35 279 113 96 1. 18 .030 4.0 Do.

Inconel X 11.5 153 290 68 76 0. 90 .090 0.5 Smooth shiny sur- PRODUCTS UNAOGEPTABLE 11 17-7 pH Steel..-- 17.1 199 204 113 109 1.04 .042 1.0 Extremely uneven I I a surface with deep c annals. 12 do 11.7 71 320 113 111 1.02 .017 2.0 Rough pitted sur- 00. 13 .410 11.9 100 93' 113 63 1.79 ,023 2.0 Wfavy, scaly-surace.

ture of. airframe structural panels 'having 'maximum strength-to-weight ratios. In the latter instance, the structural panels are preformed to the desired curvature or shape and then the areas undergoing the least mechanical stress are chemically milled out to provide thinner sections.

. 6 Besides being able to control "the quality of the chemi- 1 cally milled'surface, the process has'many other advantages; One of these is that the milling solution does not have to: be discarded periodically and replaced by a new one as was the case in'thefpriorart processes. This reduction in handling constitutes a'considerable saving in both time and the economics of the process. Another advantage of this process: is that the disposal problem of waste acids has been overcome'since the only by-product is the metal phosphates such as phosphates of nickel irony and chromium, which are removed as a precipitate;

The examples illustrate that certain ;variations maybe made in carrying out the process of this invention, especially regard to the concentration of the various components of the chemical Solution. However, it is seen that certain limits in concentrations of the various ingredients must be maintained since, as seen in No. 13 of the table, for instance, too low a concentration of iron results in an unacceptable chemically milled surface. This example illustrates that one cannot expect to obtain a good chemically milled surface bystarting with the and unsatisfactory results would be obtained. When, however, the concentrations are maintained constant by a recycling process, described in this case, satisfactory and predictable results can be obtained. Certain other variations may be made in carrying, out the process of this inventionwithout departing from thespirit and scope of the chemical milling process as described hereinabove. For example, the amount oif water that is added to the precipitation container 17 may vary from one. to ten times the volume, of the phosphates of iron residue admitted from the distillation apparatus 8,

The acids used in this process were commercially available. The phosphoric acid used was either 85 percent acid by weight having a specific gravity of 1.69, or 75 percent acid by weight having a specificgravity of 1.5.8.

The nitric acid employed was 70 percent by weight acid and had a specific gravity of 1.42. Two kinds of hydrochloric acid were used, of which one was 37 percent acid by weight having a specific gravity of 1.18, and the other was 30 percent acid by weight and had a specific gravity of 1.15.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustrationand example only and is not to be taken byway of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim: a

1. The continuous process of chemically milling workpieces of group VIII metals and alloys thereof comprising exposing a workpiece to the etching action of a bath initially containing dissolved phosphorus and metal in a predetermined weight ratio of from about 0.23 to about 1.50 phosphorus-to-metal, wherein the phosphorus is present in the form of soluble phosphates, withdrawing partially spent portions of said bath containing said dissolved metal and phosphorus, precipitating metal phosphates from said portions to form a precipitate of metal phosphate and an elfiuent containing metal hydrogen phosphates, returning said efiiuent to said bath, and adding phosphorus in the form of phosphates to said bath in an amount substantially equivalent to the phosphorus removed in said partially spent portions and maintaining said dissolved metal and phosphorus at said predetermined weight ratio.

2. A continuous process of chemically milling workpieces of group VIII metalsand alloys thereof comprising exposing a workpiece to the etching action of a bath initially containing soluble phosphates and metal in a predetermined weight ratio oi phosphorus-to-metal, withdrawing partially spent portions of said ba-th, precipitating metal phosphates from said portions to form a precipitate of metal phosphates and an efiluent containing metal hydrogen phosphates, returning said effluent containing metal hydrogen phosphates to said bath, and adding phosphorus in the form of soluble phosphates to said bath in an amount equivalent to the phosphorus removed in said metal phosphate precipitate and maintaining said dissolved metal and phosphorus at said predetermined weight ratio.

3. A process for chemically milling workpieces of group metal weight ratio is from about 0.23 to about 1.50;

.10 ing said dissolved metal and phosphorus at said predetermin-ed weight ratio.

4. The process of claim 3 wherein said bath contains substantially 35-123 grams per liter of acidic hydrogen, 35.5-302 grams per liter of chloride ion, 80.5-290 grams per liter of nitrate ion, 43-172 grams per liter of phosphorus in the form of soluble phosphates, 30-200 grams per liter of dissolved metal, the balance being substantially Water, and wherein the ratio by weight of phosphorus-to-metalin solution is maintained at from about 0.23 to about 1.50. a

5. T he continuous process of chemically milling workpieces of steel and alloys thereof comprising exposing a workpiece to the etching action of a bath initially containing substantially 3.5-12.3 grams per liter of acidic hydrogen, 35.5-302 grams per liter of chloride ion, 80.5- 279 grams per liter of nitrate ion, 43-172 grams per liter of phosphorus in the form of soluble phosphates, 30-200 grams per liter of dissolved metal, the balance being substantiallywater, and wherein the ratio by weight of phosphorus-to-metal in solution is from about 0.23 to about 'l.50, withdrawing-partially spent portions of said bath,

progressively distilling said withdrawn portions to produce a distillate of water, a distillate of hydrochloric plus nitric acids, anda residue containing phosphates of metal; adding-water to said residue to precipitate metal phosphates leaving a liquid phase containing metal hydrogen phosphates, separating said liquid phase from said precipitate, returning said liquid phase containing metal hydrogen phosphates to said bath, and adding phosphorus in the form-of soluble phosphates to said bath in an amount equivalent to the phosphorus removed in said metal phosphate precipitate and maintaining said dissolved metal and phosphorus at said predetermined weight ratio. r i.

6. The continuousprocess of chemically milling workpieces of steel and alloysthereof comprisingexposing a workpiece to the etching action or a bath initially containing substantially 3.5-1213grams per liter of acidic hydrogen, 35.51-302 grams per liter of chloride ion, 80.5- 279igrams per liter of nitrate ion, 43-172 grams per liter of phosphorus in the form of soluble phosphates, 30-200 grams'per literof dissolved metal, the balance being substantially water, and whereinthe ratio by weight of phosphorus-to-metal in solution is from about 0.23 to about 1.50, withdrawing partially spent portions of said bath, progressively. distilling said withdrawn portion to produce aydistillate of water, a distillate of hydrochloric plus nitric acids, and a residue containing phosphates of metal; ad-ding water-to said residue to precipitate metal phosphateslleavinga liquid phase containing metal hydrogen phosphates; separating saidliquid phase from said precipitate, concentrating saidliquid phase containing said metalhydrogen phosphates by removing excess wa-ter, returning .said liquid phase containing metal hydrogen phosphates to said bath, and adding phosphorus in the form of soluble phosphates to said bath in an amount equivalent to. the phosphorus removed in said metal phosphates precipitate and maintaining said dissolved metal and phosphorus at said predetermined weight ratio.

7. The process of claim 6 and in addition the steps of adding chloride ion, nitrate ion and water to said bath inan" amount substantially equivalent to that lost in carrying out said process.

8; The process of chemically milling a metallic material selected from the class consisting of a group VIII metaliand an alloy of ag'roupVIII metal and by subjecting said metallicmaterial to the action of a bath initially containing hydrochloric; nitric and phosphoric acids-and dissolved metal wherein the phosphorus-tometal weight ratio is maintained withina predetermined range of values; thesteps of withdrawing partially spent portions. of said bath containing dissolved phosphates and metal, distilling off the unused acids leaving metal phosphates and metal hydrogen phosphates in solution,

' 11 diluting said solution and thereby precipitating metal phosphates and leaving metal'hydrogen phosphates in solution, returning the metal hydrogen phosphates to the bath to maintain said phosphorus-to-metal weight ratio.

9. The process of chemically milling group VIII metals and alloys thereof by subjecting said metals and alloys to the action of a bath initially containing hydrochloric, nitric and phosphoric acids, and dissolved metal in amounts suflicient to maintain a phosphorus-to-inetal weight ratio of from about 0.23 to about 1.50, the steps of withdrawing partially spent portions of said bath containing dissolved phosphates and metal, distilling off the unused hydrochloric and nitric acids leaving metal phos phates and metal hydrogen phosphates, diluting said phosphates and thereby precipitating metal phosphates leaving metal hydrogen phosphates in solution, and returning the metal hydrogen phosphates to said bath to maintain said phosphorus-to-metal weight ratio.

10. The process of chemically milling group VIII metals and alloys thereof by subjecting said metals to the action of a bath initially containing from about 35 .5- 302 grams per liter of chloride ion, 80.5-290 grams per liter of nitrate ion, 43-172 grams per liter of phosphorus in the form of soluble phosphates, 30-200 grams per liter of dissolved metal, the balance being substantially water and wherein the ratio by weight of phosphorusto-metal is from about 0.23 to about 1.50, the steps of withdrawing partially spent portions of said bath, distilling oil the unused hydrochloric and nitric acids leaving metal phosphates and metal hydrogen phosphates, diluting said phosphates and thereby precipitating metal phosphates leaving metal hydrogen phosphates in solution, and returning the metal hydrogen phosphates to the bath to maintain said phosphonus-to-metal weight ratio, and adding phosphorus in the form of phosphates to said bath in an amount substantially equivalent to the phosphorus removed in the form of metal phosphates.

11. The process of claim and in addition the steps of adding chloride ion, nitrate ion and water to said bath in an amount substantially equivalent to that lost in carrying out said process.

12. The process of chemically milling a metallic material selected from the class consisting of a group VIII metal and an alloy of a group VIII metal by subjecting said metallic material to the action of a bath initially containing substantially 92 grams per liter of chloride ion, substantially 148 grams per liter of nitrate ion, substantially 113 grams per liter of phosphorus in the form of soluble phosphates, substantially 106 grams per liter of dissolved metal, the balance being substantially water and wherein the ratio by weight of phosphorus-to-metal is substantially 1.08, the steps of withdrawing partially spent portions of said bath, distilling oif the unused hydrochloric and nitric acids leaving a residue of metal,

phosphates and metal hydrogen phosphates, adding water to said phosphates and thereby precipitating metal phosphates leaving metal hydrogen phosphates insolution, returning said metal hydrogen phosphates to said bath and adding phosphorus in the form of soluble phosphates to said bath in an amount substantially equivalent to the phosphorus removed in the form of metal phosphates to maintain said phosphorus-to-metal weight ratio and adding chloride ion, nitrate ion and water in an amount substantially equivalent to that lost in carrying out said process.

13. The process of chemically milling a metallic material selected from the class consisting of nickel and an alloy of nickel by subjecting said metallic material to the action of 'abath initially containing substantially j 153 grams per liter"of chloride ion, substantially 290 grams per liter of nitrate ion, substantially 68 grams per liter of'phosphoru's in the'form of soluble phosphates,

' substantially 76 grams per liter of dissolved metal, the

balance being, substantially water. and wherein the ratio;

by weight of phosphorus-to-metal is substantially 0.90,

the steps of withdrawing 7 bath, distilling oil? the unused hydrochloric and nitric partially spent portions of said acids leaving a residue of metal phosphates and metal hydrogen phosphates, adding water to said phosphates and thereby precipitating metal phosphates leaving metal hydrogen phosphates in solution, returning the metal hydrogen phosphates to said bath and adding phosphorus in the form of phosphates to said bath in an amount substantially equivalent to the phosphorus removed in the form of metal phosphates to maintain said phosphorusto-metal weight ratio.

14. A continuous process for chemically milling a workpiece selected from the class consisting of steel and an alloy of steel comprising exposing said workpiece to the etching action of a bath initially containing substantially 9.6 grams per liter of acidic hydrogen, substantially 92 grams per liter of chloride ion, substantially 148 grams per liter of nitrate ion, substantially 113 grams per liter of phosphorus in the form of soluble phosphates, substantially 106 grams per liter of dissolved metal, the balance beingsubstantially water, and whe'rein the ratio by weight. of phosphorus-to-metal in solution .is substantially 1.08, withdrawing partially spent portions 25 :tions to produce a distillate of water, a distillate of hyof said bath; progressively distilling said withdrawn pordrochloric and nitric acids, and a residue containing phosphates of metal; adding water to said residue and thereby precipitating metal phosphates leaving a liquid phase containing metal hydrogen phosphates, separating said liquid phase from said precipitate, concentrating said liquid phase containing said metal hydrogen phosphates by removing excess water, returning said concentrated liquid phase containing metal hydrogen phosphates to said bath, adding phosphorus in the form of soluble phosphates to said bath in an amount equivalent to the phosphorus removed in said metal phosphate precipitate and maintaining said phosphorus-to-metal in said bath at said predetermined weight ratio; and adding chloride ion, nitrate ion and water to the said bath in an amount substantially equivalent to that lost in carrying out said process.

15. A continuous process for chemically milling a workpiece selected from the class consisting of nickel and an alloy of nickel comprising exposing said workpiece to the etching action of a bath initially containing substantially 3.5-12.3 grams per liter of acidic hydrogen, 35.5302 grams per liter of chloride ion, 80.5-290 grams per liter of phosphorus in the form of soluble phosphates, 30-200 grams per liter of dissolved metal, the balance being substantially water, and wherein the ratio by Weight of phosphorus-to-metal in solution is within the range of values of from about 0.23 to about 1.50, withdrawing partially spent portions of said bath; progressively distilling said Withdrawn portions to produce a distillate of water, a distillate of hydrochloric plus nitric acids, and a residue containing phosphates of metal; adding water to said residue and thereby precipitating metal phosphates leaving a liquid phase containing metal hydrogen phosphates, separating said liquid phase from said precipitate, returning said liquid phase containing metal hydrogen phosphates to said bath, and adding phosphorus in the form of soluble phosphates to said bath in anamount equivalent to the phosphorus removed in said metal phosper liter of phosphorus in the form of soluble phosphates,

substantially 76 grams per liter of dissolved metal, the balance being substantially water, and wherein the ratio by weight of phosphorus-to-metal in solution is substantially 0.9, withdrawing partially spent portions of said bath; progressively distilling said withdrawn portions to produce a distillate of water, a distillate of hydrochloric plus nitric acids, and a residue containing phosphates of metal; adding water to said residue and thereby percipitating metal phosphates leaving a liquid phase containing metal hydrogen phosphates, separating said liquid phase from said precipitate, concentrating said liquid phase containing said metal hydrogen phosphates by removing excess water, returning said concentrated liquid phase containing said metal hydrogen phosphates to said bath, and adding phosphorus in the form of soluble phosphates to said bath in an amount equivalent to the phosphorus re- 14 moved in said phosphate metal precipitate, and maintaining said ratio by weight of phosphorus-to-metal in said bath.

17. The process of claim 16 and in addition the steps of adding chloride ion, nitrate ion and. water to said bath in an amount substantially equivalent to that lost in carrying out said process.

References Cited in the file of this patent UNITED STATES PATENTS 2,235,658 Waterman Mar. 18, 1941 2,337,062 Page Dec. 21, 1943 2,662,814 Swihart Dec. 15, 1953 

1. THE CONTINUOUS PROCESS OF CHEMICALLY MILLING WORKPIECES OF GROUP VIII METALS AND ALLOYS THEREOF COMPRISING EXPOSING A WORKPIECE TO THE ETCHING ACTION OF A BATH INITIALLY CONTAINING DISSOLVED PHOSPHORUS AND METAL IN A PREDETERMINED WEIGHT RATIO OF FROM ABOUT 0.23 TO ABOUT 1.50 PHOSPHORUS-TO-METAL, WHEREIN THE PHOSPHORUS IS PRESENT IN THE FORM OF SOLUBLE PHOSPHATS, WITHDRAWING PARTIALLY SPENT PORTIONS OF SAID BATH CONTINIG SAID DISSOLVED METAL AND PHOSPHORUS, PRECIPITATING METAL PHOSPHATES FROM SAID PORTIONS TO FORM A PRECIPTATE OF METAL PHOSPHATE AND AN EFFUENT CONTAINING METAL HYDROGEN PHOSPHATES, RETURNING SAID EFFUENT TO SAID BATH, AND ADDING PHOSPHORUS IN THE FORM OF PHOSPHATES TO SAID BATH IN AN AMOUNT SUBSTANTIALLY EQUIVALENT TO THE PHOSPHORUS REMOVED IN SAID PARTIALLY SPENT PORTIONS AND MAINTAINING SAID DISSOLVED METAL AND PHOSPHORUS AT SAID PREDETERMINED WEIGHT RATIO. 